The dopamine transporter (DAT) has been a principal brain receptor site that has been correlated with the rewarding and euphoric properties of cocaine. MNB scientists cloned the DAT cDNA and gene, and found that deletion of both DAT and SERT are required to eliminate cocaine conditioned place preferences in mice. DAT is required for the actions of each of the current dopamine-selective toxins that produce the best models of Parkinson's disease. Analyses of DAT structure- function relationships, and their relationships with SERT, continued during this year with further characterization of the roles of selected amino acid sidechains in transporter functions. These studies have focused on single- and multiple-domaine amino acid changes of interest in the dopamine transporter and on characterization of the amino acid changes produced by human allelic variants of the DAT sequence identified in other studies reported during this year. Studies ahve identified human 5' and 3' DAT haplotypes and have worked to correlate common 5' haplotype variants with levels of DAT expression in vivo. Studies reported during this year document surprisingly large effects of serine and threonine substitutions on the DAT activity effects of drugs that activate or inhibit PKC, MAP, MEK kinase, and IP3 kinase pathways. Studies completed during this year have also identified a large effect of coexpression of the PKC-dependent PP1 inhibitor, KEPI (identified in this laboratory as a morphine-upregulated gene)on the function of coexpressed DAT. Studies completed during this year also document surprisingly-selective effects of DAT point mutations on efflux of the Parkinson's disease producing toxin MPP+ and dopamine from cells expressing DAT and its mutants. These insights should continue to help in identification of structure- function features of small molecule compounds possibly active in vivo as cocaine antagonists, structure-function relationships relevant to DAT regulation, and human individual differences in DAT levels of expression and pharmacology.